It is known to treat pyrogenically produced oxides, oxide mixtures or mixed oxides of metals and/or metalloids with hydrophobing agents, for example dimethyldichlorosilane (DE AS 11 63 784).
It is also known to treat finely particulate oxides in a bed with octamethylcyclotetrasiloxane (GB-A 887 257).
It is furthermore known to render finely particulate oxides hydrophobic with octamethylcyclotetrasiloxane in a one-pot process (GB-A 932 753, U.S. Pat. No. 2 803 617).
The known processes have the disadvantage that the octamethylcyclotetrasiloxane is not completely bonded to the surface of the oxides.